    Patent Document 1: JP-A H11-080647    Patent Document 2: JP-A 2000-239853    Patent Document 3: JP-A 2006-321948    Patent Document 4: JP-A 2004-33901
Generally, metal nanoparticles have a very high ratio of the particle surface thereof to the total number of atoms and have physical properties different from those of bulk metal. Particularly, metal nanoparticles having a diameter of 10 nm or less exhibit a high catalyst function and undergo a reduction in melting point, and in the case of gold, silver, copper, or the like show a physical property of having absorption at a specific wavelength, called plasmon absorption in the visible light range.
A solution having such metal microparticles dispersed uniformly therein, that is, a metal colloidal solution, has been utilized in various fields by making use of its characteristics. For example, the metal colloidal solution can be utilized as a colorant in paints etc. (JP-A H11-080647/Patent Document 1) and in production of a thin film having metallic luster (JP-A 2000-239853/Patent Document 2) or can be utilized in an electroconductive paste with which electrodes and circuits in chip parts, plasma display panels etc. are made significantly fine and formed or printed with high density, high accuracy, and high reliability.
Among previous methods for producing metal colloidal solutions, there are those methods carried out with a batch reaction apparatus as shown in JP-A 2006-321948 (Patent Document 3) or with a general micro-reactor as shown in JP-A 2004-33901 (Patent Document 4). In the batch type method, however, it is generally difficult to control the temperature in the batch reaction apparatus and it is thus inevitably difficult to conduct a uniform reaction. In addition, the concentration control is also difficult because complete uniform stirring is necessary. Further, a long reaction time is also necessary, and thus it is extremely difficult to control all the reaction conditions. Although there are many advantages in micro-devices and system when the general micro-reactor is used, there are actually many problems as follows: when the diameter of a micro-flow path is decreased, the pressure loss is inversely proportional to the fourth root of the flow path, that is, a fluid-sending pressure that is so high as to make it hard to obtain a pump for sending a fluid is necessary; in the case of a reaction involving separation, the phenomenon of clogging a flow path with products and the closure of a micro-flow path by foam generated by the reaction; the reaction is expected fundamentally by the diffusion speed of molecules, and thus the micro-spaces are not effective or applicable to every reaction, and in practice, successful ones should be selected by testing the reaction in a trial-and-error system.
Scaling up has been coped with a method of increasing the number of microreactors, that is numbering up, but the number of microreactors which can be stuck is limited to several dozen, thus inherently aiming exclusively at products of high value, and the increase in the number of devices leads to an increase the absolute number of failure causes, and when the problem of clogging actually occurs, it can be very difficult to detect a problem site such as failure site.